Landform Analysis, Vol. 17: 225–227 (2011)
Impact of upstream sediment inflow on headcut
morphodynamics
Robert R. Wells
1, Sean J. Bennett
2, Carlos V. Alonso
3 1USDA-ARS National Sedimentation Laboratory USA2Department of Geography, University at Buffalo,USA
3USDA-ARS National Sedimentation Laboratory (retired), USA e-mail: robert.wells@ars.usda.gov
Abstract: Headcut erosion can severely accelerate soil loss in upland concentrated flows and lead to significant soil
degra-dation in agricultural areas. Previous experimental work has demonstrated that actively migrating headcuts display system-atic morphodynamic behavior, and impinging jet theory can provide an excellent theoretical foundation for this erosional phenomenon. This research sought to examine systematically the effect of an upstream sediment inflow on the morphodynamics of actively migrating headcuts in upland concentrated flows. Using a specially designed experimental facil-ity, actively migrating headcuts were allowed to develop, and then subjected to an upstream sediment load composed of sand. As the upstream sediment feed rate increased, the size and migration rate of the headcut decreased markedly, but sedi-ment discharge was less affected. The headcut erosion process was arrested as sedisedi-ment inflow rates increased above a threshold value. As sediment feed rate upstream of the headcut increased, sediment size fraction downstream of the headcut also increased. This research suggests that headcut erosion can be greatly modulated by an upstream sediment source, fur-ther complicating the prediction of soil erosion on upland areas.
Keywords: headcuts, overland flow, simulated rainfall, soil erosion
Introduction
An experimental program was initiated to exam-ine actively migrating headcuts in concentrated flows and to address this soil erosion phenomenon in mech-anistic terms. To date this research has documented the following observations: (1) steady-state soil ero-sion can be achieved under specific conditions; (2) larger scour holes are associated with higher overland flow rates, higher bed slopes, and larger initial step heights; (3) the presence of a non-erodible layer re-duces scour depths, nappe entry angles, and sediment discharges; (4) higher tailwater heights downstream of the headcut lead to an immediate cessation of the soil erosion process; and (5) varying subsurface pore-water pressures can either enhance or suppress the headcut erosion process (Bennett 1999, Bennett et al. 2000, Bennett & Casalí 2001, Gordon et al. 2007, Wells et al. 2009a, b).
These observations were made using clear-water flow as the upstream boundary condition. It is well
known that the presence of sediment in transport within upland concentrated flows can markedly af-fect flow hydraulics and flow resistance (Li & Abrahams 1997). The goal of this research was to systematically examine the effect of upstream sedi-ment inflow on the morphodynamics of actively mi-grating headcuts in upland concentrated flows.
Methods
The present study consisted of 14 experiments: 2 clear-water (baseline) runs, and 12 runs with an up-stream sediment feed. Six sediment (0.35-mm me-dian grain size) inflow rates were imposed based on the observed sediment discharge rate Qs for the
baseline experiments (0.2Qs, 0.4Qs, 0.6Qs, 0.8Qs,
1.0Qs, and 1.2Qs), and each sediment-feed
experi-ment was replicated. All experiexperi-ments were con-ducted in a 5.5-m long and 0.165-m wide non-recir-culating, tilting hydraulic flume. This facility and the 225
procedures employed to apply rainfall and monitor water surface heights, runoff, and headcut erosion processes were described in detail by Wells et al. (2009a, b).
The soil used in the present study was an Atwood sandy clay loam (fine-silty, mixed, thermic Typic
Paleudalfs) with 72% sand, 11% silt, and 17% clay.
The median grain size of the sand fraction within the Atwood soil is 0.25 mm in diameter. Water and sedi-ment exiting the soil cavity were captured in 0.5-L glass bottles at 10-s intervals for the first 3 minutes then 20-s intervals thereafter. Sediment samples were weighed and placed in an oven at 40.5°C for 24 hr, then reweighed to determine sediment concen-tration.
The dry sediment samples were sieved to deter-mine the percent mass of size for each of the follow-ing size fractions (all diameters in mm): coarser than 0.5, 0.354, 0.25, 0.178, 0.125, 0.088, 0.063, and less than 0.063 (pan remains). These fractions were com-bined into the following particle size bins: medium sand (coarser than 0.354 mm), fine sand (0.178 to 0.354 mm), very fine sand (0.063 to 0.178 mm), and silt and clay (finer than 0.063 mm).
Results
Initial headcut growth and development oc-curred at the imposed step-change in bed surface
to-pography. Following an initial period of bed adjust-ment, steady-state erosion conditions were achieved: a headcut migrated upstream at a nearly constant rate and shape, and the sediment discharge exiting the flume was also nearly invariant. Approximately 120 s after the development of an actively migrating headcut, sediment feed was initiated.
It was observed that the headcut migration rate (M) slowed, the maximum scour depth (SD)
de-creased, and the jet entry angle (èe) decreased (Table
1). For sediment feed rates of 0.8Qsand higher, the
actively migrating headcut was completely obliter-ated given enough time and space. That is, as the up-stream sediment feed rate approached the sediment efflux generated through headcut scour and migra-tion, the headcut itself ceased to exist.
Two additional points are contended here. First, sediment influx of 0.2Qsand smaller had no effect on
headcut morphodynamics. For relatively small sedi-ment influxes, it is suggested that these flows were detachment capacity-limited, and these loadings, therefore, were of little consequence to the overland flow hydraulics and scour pool hydrodynamics. Sec-ond, sediment influxes of 0.4Qsand higher would
re-sult in the slowing or cessation of headcut migration under the slope and overland flow discharge condi-tions imposed in these experiments.
The time variation of sediment discharge did not mirror the morphodynamic behavior of the cuts. Following the initial bed adjustment and
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Robert R. Wells, Sean J. Bennett, Carlos V. Alonso
Table 1. Summary of experimental parameters: M (migration rate), SD(maximum scour depth), θe(nappe entry angle), and Qs(sediment discharge) were determined prior to sediment addition
Run Sediment Influx s Q M SD e Qs
Units kg s–1 kg m–3 m3s–1 mm s–1 m deg kg s–1 Baseline 0 1288 0.00114 2.4 0.092 58 0.0223 Baseline 0 1265 0.00120 2.3 0.089 43 0.0144 0.2Qs 0.0047 1307 0.00116 1.9 0.096 47 0.0254 0.2Qs 0.0047 1296 0.00119 1.7 0.096 49 0.0227 0.2Qs 0.0047 1331 0.00112 1.9 0.091 47 0.0223 0.4Qs 0.0094 1339 0.00109 2.8 0.087 41 0.0278 0.4Qs 0.0094 1337 0.00114 2.8 0.085 43 0.0229 0.6Qs 0.0142 1339 0.001161 2.7 0.076 42 0.0234 0.6Qs 0.0142 1339 0.00116 2.6 0.085 45 0.0227 0.8Qs 0.0189 1327 0.00110 2.8 0.084 45 0.0316 0.8Qs 0.0189 1270 0.00122 2.8 0.086 42 0.0217 1.0Qs 0.0236 1345 0.00112 2.9 0.079 46 0.0345 1.0Qs 0.0236 1759 0.00114 3.0 0.081 42 0.0210 1.2Qs 0.0283 1325 0.00115 2.5 0.088 40 0.0326 1.2Qs 0.0283 1351 0.00116 2.8 0.092 41 0.0235
cut growth and development, all Qsdata show an
as-ymptotic decline with time. While M, SD, and èeall
decline with time for sediment influxes of 0.4Qsand
higher, no such changes were noted in the Qsdata.
That is, while the actively migrating headcuts be-came smaller with time, or in some cases ceased to exist, the sediment discharge rates remained nearly invariant with time and fairly constant amongst the experiments.The texture of the sediment efflux within the experiments, however, was affected by the sediment influx. The texture of the sediment efflux for the baseline experiment was dominated by the fine sand fraction since this fraction dominates the composition of the soil material, and no discernable time variation in texture was observed in the mass fractions of Qsfor the baseline experiment. Similar
textural composition and time variation was ob-served for the 0.2Qsexperiment. Yet as the sediment
influx increased above this value, a marked shift oc-curred in the texture of the sediment efflux. The mass fraction of medium sand increased with time and the fine and very fine sand and silt and clay frac-tions decreased with time.
Conclusions
Research has focused on quantifying the mor-phodynamics of soil erosion due to headcut migra-tion on unconstrained hillslopes and agricultural fields. This study sought to examine the effect of up-stream sediment inflow on the growth and migration of headcuts in concentrated flows. Clear-water ex-periments result in steady-state soil erosion wherein a headcut develops and enlarges due to the imposed flow rate, but attains a constant rate of migration, shape, and sediment discharge as a function of time. As the sediment inflow rate of fine sand increases above a certain threshold, the size and migration rate of the headcut decrease markedly, thus arresting lo-cal soil erosion. Sediment discharge, in turn, shifts
from headcut-controlled flux to sediment-feed flux. The progressive obliteration of the headcut pool demonstrates that headcut migration is greatly mod-ulated by upstream sand inflow, which renders inad-equate the use of steady-state, algebraic models. Thus, more comprehensive headcut-erosion predic-tors are needed that treat this phenomenon as an ini-tial, boundary-value problem solved with fast numer-ical engines.
References
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